1. Field of the Invention
The present invention relates to a brake control system for an electrically operated vehicle to perform a regenerative braking operation and a hydraulic braking operation, and more particularly to the brake control system which is adapted to add the hydraulic braking operation to the regenerative braking operation, or change the former for the latter.
2. Description of the Related Arts
In an electrically operated vehicle provided with an electric motor as a power source, the electric motor also functions as a generator for performing the regenerative braking operation to recover energy by charging a battery and use the energy for driving the motor. Since the braking force applied to the vehicle wheels through the regenerative braking operation is limited, it must be compensated by a hydraulic braking operation. Therefore, both of the regenerative braking operation and the hydraulic braking operation are performed, in general. In Japanese Patent Laid-open Publication No. 5-176406, for example, a brake control apparatus is proposed to perform a proper braking control which is adapted to comply with a difference between the maximum regenerative braking force which is provided when a vehicle travels at a relatively high speed and the maximum regenerative braking force which is provided when the vehicle travels at a relatively low speed. The apparatus includes a plurality of cut-off means which are disposed in series on a pressure transmitting passage for communicating a pressure generator to a wheel cylinder operatively mounted on a driven wheel, to shut off the hydraulic pressure generated by the pressure generator until it reaches a predetermined value. The apparatus further includes communication changeover means for selectively bypassing the cut-off means in response to a rotational speed of a motor to change the communication between the cut-off means. In practice, when the vehicle travels at the relatively high speed, the hydraulic braking operation is made to provide the hydraulic pressure corresponding to a difference .DELTA.P exceeding a certain pressure to enable only a pressure decreasing valve 30 to be opened. When the vehicle travels at the relatively low speed, the hydraulic braking operation is made to provide the hydraulic pressure corresponding to the difference .DELTA.P exceeding a certain pressure to enable both of the pressure decreasing valves 30 and 32 to be opened.
According to a brake apparatus disclosed in Japanese Patent Laid-open Publication No. 7-336806, an ON/OFF valve is disposed between the master cylinder and the wheel brake cylinder, and closed when the regenerative braking operation is performed. In the Publication, it was raised as an issue to be solved that a brake pedal was moved forward to vary the depressing force, when the ON/OFF valve was switched from its closed condition to its open condition so as to cancel a mode for giving a priority to the regenerative braking operation. Therefore, the brake apparatus was proposed to rapidly increase the pressure in a hydraulic braking device by temporarily introducing a hydraulic pressure from a hydraulic booster. When the braking operation is switched from the regenerative braking operation to the hydraulic braking operation, the master cylinder pressure is temporarily reduced until the wheel cylinder pressure is increased close to the master cylinder pressure, so that the stroke of the brake pedal is rapidly varied and the vibration of the depressing force occurs to deteriorate the brake pedal feeling, as described in the Publication No. 7-336806. According to the apparatus proposed in the Publication, when the difference of the pressure between relief valves (VR1), (VR2) is to be cleared, a solenoid valve (V1) is closed at the outset and subsequently a solenoid valve (V4) is opened to temporarily introduce the hydraulic pressure from a hydraulic booster (HB), so as to prevent a large pressure difference from being caused between the master cylinder pressure and the wheel cylinder pressure.
According to the apparatus proposed in the Publication No. 5-176406, however, the hydraulic braking operation is made to provide the hydraulic pressure corresponding to the difference .DELTA.P exceeding the pressure to enable both of the pressure decreasing valves 30 and 32 to be opened, and a pair of pressure decreasing valves are arranged in series, so that its response characteristic is to be improved.
According to the apparatus proposed in the Publication No. 7-336806, the hydraulic booster (HB) included in a dynamic pressure circuit is connected to a static pressure circuit between the master cylinder and the wheel brake cylinder through the solenoid valve (V4). The static pressure circuit includes the relief valve (VR2) for use in adding the hydraulic braking operation to the regenerative braking operation or changing the former for the latter, a solenoid valve (V5) and a check valve (VC2). Therefore, when the hydraulic braking operation is changed for the regenerative braking operation, brake fluid is supplied from the hydraulic booster (HB) to the static pressure circuit, so that the brake fluid in the static pressure circuit is not compensated, even if a brake pedal is depressed. As a result, a feeling of a vehicle driver in his braking operation will be deteriorated.